The following abbreviations are herewith defined:    3GPP third generation partnership project    ARQ automatic repeat request    BLER block error ratio    C/I carrier to interference ratio    CQI channel quality indicator    DL downlink    HARQ hybrid ARQ    LTE long term evolution    Node B base station    eNB EUTRAN Node B    OFDMA orthogonal frequency division multiple access    PRB physical resource block    PS Packet Scheduler    TTI transmission timing interval    UL uplink    UE user equipment    UTRAN universal terrestrial radio access network    EUTRAN evolved UTRAN    aGW access gateway
A proposed communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE) is at present a study item within the 3GPP. The current working assumption is that the access technique will be OFDMA for the DL, which can be expected to provide an opportunity to perform link adaptation and user multiplexing in the frequency domain.
In an E-UTRAN system signal retransmission is implemented in an Hybrid Automatic Repeat Request (“HARQ”) process. As is described in section 9.1 of 3GPP TS 36.300 Technical Report, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA), in general HARQ can be classified as being synchronous or asynchronous.
Synchronous HARQ implies that (re)transmissions for a certain HARQ process are restricted to occur at known time instants. No explicit signaling of the HARQ process number is required as the process number can be derived from, e.g., the subframe number.
Asynchronous HARQ implies that (re)transmission for a certain HARQ process may occur at any time. Explicit signaling of the HARQ process number is therefore required. In principle, synchronous operation with an arbitrary number of simultaneous active processes at a time instant could be envisioned. In this case, additional signaling may be required. Asynchronous operation already supports an arbitrary number of simultaneous active processes at a time instant. Furthermore, note that, in a synchronous scheme the transmitter may choose not to utilize all possible retransmission instants, e.g., to support pre-emption. This may require additional signaling.
The various forms of HARQ are further classified as adaptive or non-adaptive in terms of transmission attributes, e.g., the resource unit (RU) allocation, modulation and transport block size, and the duration of the retransmission. Control channel requirements can be different for each case.
Adaptive HARQ implies that the transmitter may change some or all of the transmission attributes used in each retransmission, as compared to the initial transmissions (e.g. due to changes in the radio conditions). Hence, the associated control information needs to be transmitted with the retransmission. The changes considered are: modulation, resource unit allocation and duration of transmission.
Non-adaptive HARQ implies that changes, if any, in the transmission attributes for the retransmissions are known to both the transmitter and receiver at the time of the initial transmission. Hence, associated control information need not be transmitted for the retransmission.
The capability to adaptively change the packet format (i.e., adaptive IR) and the transmission timing (i.e., asynchronous IR) yields an adaptive, asynchronous IR based HARQ operation. Such a scheme has the potential of optimally allocating the retransmission resources in a time varying channel. For each HARQ retransmission, control information about the packet format needs to be transmitted together with the data sub-packet.
The various types of HARQ retransmission processes can have a significant impact on the complexity of user equipment and other electronic devices that are configured to receive HARQ retransmissions. In particular, in order to support IR HARQ, where retransmissions can contain different channel bits than the original transmissions, the UE needs to have a large buffer that can store all the channel bits for all the HARQ processes that are involved. In UE intended to operate in an EUTRAN wireless communications system, this buffer in conventional systems is very large and comprises a significant part of the modem application-specific integrated circuit (“ASIC”). This may require a 10 Mbit buffer occupying a region 15 mm2 on the die of the ASIC.
Various conventional techniques have been proposed to reduce HARQ buffering requirements. In one such scheme implemented in an HSDPA modem the size of the buffer was reduced by only allowing chase combining. The performance of this method has been found to be lacking. It is also known that the size of the buffer can be reduced by taking the probability of buffer overflow into account. Problems have been found in such methods associated with buffer segmentation when receiving different transport block sizes. Also, having one CRC per turbo code block instead of per transport block has been proposed for power saving reasons.
Each of these conventional methods have limitations, and those skilled in the art seek apparatus, methods and computer program products that overcome the limitations of the prior art.